In lithography, an exposure energy, such as ultraviolet light, is passed through a mask and onto a target such as a silicon wafer. The mask contains opaque and transparent regions formed in a predetermined pattern. The exposure energy exposes the mask pattern on a layer of resist formed on the target. The resist is then developed for removing either the exposed portions of resist for a positive resist or the unexposed portions of resist for a negative resist. This forms a resist mask. The resist mask can then be used in subsequent fabrication processes. In semiconductor manufacture such a resist mask can be used in deposition, etching, or ion implantation processes, to form integrated circuits having very small features.
One technique used in lithography is known as phase shift lithography. With phase shift lithography, the interference between waves of an exposure energy is used to overcome diffraction and improve the resolution and depth of the images projected onto the target. In phase shift lithography, the phase of the exposure energy at the target is controlled such that adjacent bright areas are formed preferably 180 (.pi.) degrees out of phase with one another. Dark regions are thus produced between the bright areas by destructive interference. This technique improves total resolution at the target and allows resolutions as fine as 0.10 .mu.m to occur.
In the past, phase shift masks have been used experimentally to print sub micron features. However, phase shift lithography is still in research and development stages and has not been used extensively for commercial volume semiconductor manufacture. One reason phase shift lithography is not widely used commercially, is the high defect density which results during its use. In general, phase shift masks are difficult to form without defects and any defects on the mask pattern can be printed onto the target.
In general there are two types of defects: bump defects and indentation defects. A bump defect comprises unwanted material that has been deposited or not removed from a region of the mask. For example, a metal bump defect, comprising chromium or another material used in forming an opaque layer can remain after the opaque layer is etched. The bump defect can also be formed of an inorganic material such as silicon dioxide (SiO.sub.2) used in forming a phase shift layer. An indentation defect comprises an area of the mask that has been undesirably removed such as by etching. With either type of defect, the defect can cause printing errors on a target such as a semiconductor wafer.
This has led to the development of methods for eliminating defects from a phase shifting masks. One such method is described in U.S. Pat. No. 5,405,721 to Pierrat, which is incorporated herein by reference. In this method an opaque layer and two different phase shift layers are formed on the mask. The opaque layer and top phase shift layer are patterned in a conventional manner and the bottom phase shift layer is patterned only to repair missing shifter defects. However, the main purpose of the bottom phase shift layer is to provide an etch stop during repair of the phase defects or during the fabrication of the mask.
The method disclosed in the above Pierrat patent is illustrated for forming an alternating aperture mask in FIGS. 1 and 2. In FIG. 1, a mask blank 8 includes: a transparent substrate 10; a bottom phase shift layer 12, a top phase shift layer 14 and an opaque layer 16. Each of the phase shift layers 12 and 14 are formed of a material having an index of refraction and a thickness to provide a combined phase shift of 2.pi. or even integral multiple thereof (i.e., 2p.pi. where p is an integer). The additive effect of the two phase shift layers 12 and 14 is thus equivalent to no phase shift. However, the phase shift layers 12 and 14 are formed of different materials to permit selective etching to remove defects. The top phase shift layer 14 can be made of a conventional phase shift material such as SiO.sub.2 whereas the bottom phase shift layer 12 can be made of a material that provides an etch stop during defect etching. For example, for a top phase shift layer 14 formed of SiO.sub.2 and a substrate 10 formed of quartz, the bottom phase shift layer 12 can be a fluoride such as MgF.sub.2, CaF.sub.2, YF.sub.3, LaF.sub.3.
Using a blank 8 formed as shown in FIG. 1, different types of phase shift masks can be fabricated (e.g., alternating aperture phase shift mask, rim phase shift mask, chromeless phase shift mask). Referring to FIG. 2, for fabricating an alternating aperture phase shift mask 17, the opaque layer 16 can be patterned and etched to form opaque light blockers 18. The top phase shift layer 14 can be patterned and etched to form an alternating pattern of phase shifters 20 and light transmission openings 22. Exposure energy directed through a phase shifter 20 is phase shifted by .pi. (or odd multiple thereof) relative to exposure energy directed through a light transmission opening 22.
An indentation defect 26 comprises an etched trench in one of the light transmission openings 22. In order to remove the indentation defect 26, a gallium ion beam is focused on the region wherein the indentation defect 26 is located. The ion beam forms by ion milling a recess 28 (indicated by dotted lines) that extends through both phase shift layers 12 and 14 to the substrate 10. The ion milling process is terminated as soon as chemical by-products, or secondary ions, resulting from the ion beam contacting the substrate 10 are detected. In use of the phase shift mask 17, the recess 28 does not phase shift the exposure energy. Accordingly, there is still a phase difference of .pi. for exposure energy directed through a recess 28 relative to exposure energy directed through a phase shifter 20.
A bump defect 24 comprises excess material left in a phase shifter 20. In this case the excess material can be the same material as the top phase shift layer 14 (e.g., SiO.sub.2). To remove the bump defect 24, the focused gallium ion beam is directed at the bump defect 24. The ion milling is terminated as soon as the detected by-products begin to shift from those known to be emitted by the material which forms the top phase shift layer 14 to those known to be emitted by the material which forms the bottom phase shift layer 12 (e.g., MgF.sub.2, CaF.sub.2, YF.sub.3, LaF.sub.3.).
With this method the ion beam may stain the area of the bottom phase shift layer 12 subjacent to the bump defect 24. In a similar manner, the ion beam may stain the area of the substrate 10 subjacent to the indentation defect 26. These stains can be removed by etching the stained areas on the substrate 10 and bottom phase shift layer 12 by an amount that can be precisely ascertained using equations that take into account the index of refraction of the different materials.
One shortcoming of this method for removing defects in a phase shift mask is that the ion milling process can be difficult to perform and control. In particular it can be difficult to precisely align and focus the gallium ion beam to the areas containing the defects. At best, this can be a time consuming process and requires expensive ion milling equipment. In addition, the subsequent stain removal process can also be difficult to perform and adds complexity to the process.
Because of these and other shortcomings in the art, there is a need for an improved method for fabricating defect-free phase shift masks. Accordingly, it is an object of the present invention to provide an improved method for fabricating defect-free phase shift masks.
It is another object of the present invention to provide an improved method for fabricating and using defect-free phase shift masks in high volume semiconductor manufacture.
It is yet another object of the present invention to provide an improved defect-free phase shift mask.
Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds.